1. Field of the Invention
The present invention relates to a surface acoustic wave device which is suitably used as a filter in a small-sized radio communication device such as a portable telephone, and in particular to a surface acoustic wave device having a balanced-unbalanced conversion function of which the input-output impedances are different from each other, and a communication device including the same.
2. Description of the Related Art
In recent years, techniques for developing small-sized, light-weight radio communication devices, such as portable telephones, have progressed a great deal. In such devices, composite parts having plural functions have been developed to reduce the number of components in the device and the size thereof.
In the above-described background, surface acoustic wave filters for use in the RF stages of portable telephones have been required to have a balanced-unbalanced conversion function, the so-called balun function. Thus, longitudinally coupled resonator type surface acoustic wave filters which perform the balanced-unbalanced signal transformation have been primarily used for band-pass filters in the RF stages of portable telephones.
The longitudinally coupled resonator type surface acoustic wave filters having a balanced-unbalanced conversion function are often connected to mixer ICs provided with balanced or differential inputs-outputs (hereinafter, referred to as a balanced type mixer IC). Influences of noise are reduced, and the output is stabilized by using the balanced type mixer IC. Thus, the characteristics of portable telephones are improved. Therefore, balanced type mixer ICs have been widely used.
In most cases, the above-described balanced type mixer ICs have a high impedance of about 100 Ω to about 200 Ω, while surface acoustic wave filters used in the RF stages usually have an impedance of about 50 Ω. Balanced type mixer ICs having an impedance of 200 Ω are primarily used. Accordingly, for longitudinally coupled resonator type surface acoustic wave devices used with balanced type mixer ICs, one of the input and output impedances must be about four-times that of the other.
To attain such input-output impedances, the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2001-267885, as shown in FIG. 28, is often used. In the configuration shown in FIG. 28, for each of longitudinally coupled resonator type surface acoustic wave elements 101 and 102, one terminal is electrically connected in parallel, and the others are connected in series.
The difference between the surface acoustic wave element 101 and the surface acoustic wave element 102 is that the phases of interdigital electrode portions (hereinafter referred to as IDT) 103 and 108 are inverted with respect to each other. Thus, the phases of the signals output through terminals 114 and 115 are different from each other by about 180°, such that an unbalanced signal input through a terminal 113 is converted to balanced signals which are output through the terminals 114 and 115.
FIG. 29 shows the frequency characteristic of the filter having the configuration of FIG. 28. FIGS. 30A and 30B show the impedance characteristics thereof. The impedance characteristic of FIG. 29 is that of the filter which is designed as an EGSM (Enhanced Global System for Mobile Communications) transmission filter. The frequency range that is required for the pass-band is from 880 MHz to 915 MHz. Points at f=880 MHz and f=915 MHz designated by X and Y, respectively, are plotted in FIGS. 30A and 30B, respectively.
As seen in FIGS. 30A and 30B, the filter is designed so as to have terminal impedances of 50 Ω on the unbalanced signal side (S11) and 200 Ω on the balanced signal side (S22). Thus, the impedances are substantially matched, such that the impedance on the balanced signal side is about four times the impedance on the unbalanced signal side.
On the other hand, some of the above-described balanced type mixer ICs have an impedance of about 100 Ω. Correspondingly, in some cases, the longitudinally coupled resonator type surface acoustic wave filter must have an impedance on the unbalanced signal terminal side that is about twice the impedance on the balanced signal terminal side.
Japanese Patent No. 3224202 discloses a filter corresponding to unbalanced-balanced input-output that is configured as shown in FIG. 31. The configuration of FIG. 31 will be described. Two longitudinally coupled resonator type surface acoustic wave elements 201 and 202 are connected to each other, in which IDT 204 of the element 201 and IDT 209 of the element 202 and also IDT 205 of the element 201 and IDT 210 of the element 202 are cascade connected to each other. A terminal 213 is an unbalanced signal terminal. A signal input through the terminal 213 is converted to signals having phases that are different from each other by about 180° in IDT 208, which are output through balanced signal terminals 214 and 215.
According to Japanese Patent No. 3224202, a desired characteristic is achieved by setting the meshing widths W of the surface acoustic wave elements 201 and 202 so as to be different from each other as shown in FIG. 31, even if the impedances on the unbalanced signal terminal side and on the balanced signal terminal side are different from each other.
However, the configuration of FIG. 31 cannot meet with the recent requirements such as a wide band, a low loss, and a high balancing degree. One of the reasons for this is that when the respective two surface acoustic wave elements 201 and 202 are cascade-connected to each other, the insertion loss is equal to the total of the insertion losses of the two elements. Moreover, since the meshing widths W in the first and second stages are different from each other, mismatching occurs between the stages. This increases the insertion loss.
By way of reference, FIG. 32 shows the frequency characteristic of one surface acoustic wave element. FIG. 33 shows the frequency characteristic of the two surface acoustic wave elements cascade-connected to each other. In the configuration of FIG. 31, the signals have phases that are different from each other by 180° in the IDT 208 and are output through the balanced signal terminals 214 and 215, respectively. However, the asymmetrical arrangement of the IDT electrodes and wirings on a substrate cannot be avoided. This affects the amplitude and phase-balance degree of the output signals. Thus, the balance degree is deteriorated as compared to that of the configuration of FIG. 28.
Accordingly, the configuration of FIG. 31 is unsuitable for a filter corresponding to unbalanced-balanced input-output which requires low-loss and a high level of balance. For such purposes, the configuration of FIG. 28 is used.
Hereinafter, the configuration of FIG. 28 will be described which includes an unbalanced signal terminal 113 provided on the input side, and balanced signal terminals 114 and 115 provided on the output side. In the configuration of FIG. 28, Ri and Ro represent the impedances of the input-output terminals of the surface acoustic wave elements 101 and 102, respectively. The impedance on the unbalanced signal terminal side is expressed by R1/2, since the terminals on the input side of the surface acoustic wave elements 101 and 102 are electrically connected in parallel to each other. The impedance on the balanced signal terminal side is expressed by 2Ro, since the terminals on the output side of the surface acoustic wave elements 101 and 102 are electrically connected in series with each other.
Ordinarily, when each of the surface acoustic wave elements 101 and 102 includes three IDTs, the impedances of the input and output terminals are approximately the same, such that Ri≈Ro. To form an unbalanced-balanced input-output filter in which the impedance on the balanced signal terminal side is about four times the impedance on the unbalanced signal terminal side, as described above, 4×Ri/2≈2Ro, that is, Ri≈Ro is required. This facilitates the design of the filter.
On the contrary, to form an unbalanced-balanced input-output filter in which the impedance on the balanced signal terminal side is about two times the impedance on the unbalanced signal terminal side, 2×Ri/2≈2Ro, that is, 2Ri≈Ro is required. Thus, it is necessary to produce the surface acoustic wave elements 101 and 102 such that 2Ri≈Ro. The design of the elements 101 and 102 is difficult.
According to one of the related art methods, a surface acoustic wave device element having an unbalanced-balanced transformation function in which the impedance on the balanced signal terminal side is about four times the impedance on the unbalanced signal terminal side is formed using surface acoustic wave elements with Ri≈Ro. To match the impedances, matching elements are provided outside the surface acoustic wave device. That is, an inductance element is provided in parallel on the balanced signal terminal side, and a capacitance element is provided in series (alternatively, the capacitance element is provided in parallel, and the inductance element is provided in parallel), such that the impedance on the balanced signal terminal side is about two times the impedance on the unbalanced signal terminal side.
FIG. 34 shows the frequency characteristic obtained when the matching is performed such that the impedance on the unbalanced signal terminal side is two times the impedance on balanced signal terminal side. FIGS. 35A and 35B show the impedance characteristics (in the range of 880 MHz to 915 MHz). FIG. 36 shows a measuring circuit in which external elements are added. It is to be noted that in FIG. 34 and FIGS. 35A and 35B, the characteristics obtained when no external element is added are shown for comparison. As seen in FIG. 34 and FIGS. 35A and 35B, the impedance on the balanced signal terminal side can be set to be about two times the impedance on the unbalanced signal terminal side by the above-described method. However, problematically, the addition of external elements increases the number of components, and hinders the development of a small-sized surface acoustic wave device.
The above-described problems are caused when the impedance on the unbalanced terminal sides is three times the impedance on the balanced terminal side as well as when the impedance on the unbalanced terminal sides is two times the impedance on the balanced terminal side.